Mineral building materials are best protected against the effects of weathering by a water-repellent impregnation and a coating applied over the latter. As soon as it is coated, a water-repellent impregnation becomes a water-repellent primer.
The primer is applied directly to the mineral substrate. By forming a hydrophobic zone, it renders the substrate water-repellent down to a certain depth and provides permanently improved union between the uniformly hydrophobicized substrate and the architectural coating. Organosilanes, oligomeric organosiloxanes or silicone resins are the best active substances for water-repellent primers.
Solvent-containing and aqueous primer compositions are used. In addition to organosilanes, oligomeric organosiloxanes or silicone resins, solvent-containing primer compositions include synthetic resins, such as styrene acrylates and pure acrylates, which serve to reinforce the substrate.
The advantage of the aqueous primer compositions is that they contain little or no organic solvent fractions. They can be used wherever the substrate is somewhat intact. The highly alkaline aqueous potassium siliconate solutions, microemulsions or emulsions of organosilanes, oligomeric organosiloxanes or silicone resins are used. The hydrophobicizing primer “levels out” the absorbency of the substrate and leads to improved adhesion of the architectural coating and to a water-repellent effect which extends into the depth of the substrate. EP-A-234 024, for example, describes the water-repellent impregnation of building materials with an emulsion of alkyl-alkoxysilanes. U.S. Pat. No. 4,757,106 describes the hydrophobicization of building materials using microemulsions of alkoxy-containing organopolysiloxanes and salts of aminosilicone oils.
The covering architectural coatings are applied in one or more coats over the water-repellent primer. Examples of such architectural coatings are paints and plasters. Pretreating the substrate with a water-repellent primer is important when the architectural coating includes emulsifiers or other wetting agents.